Devices for metal-forming by magnetic tension



Juiy 27, 1965 H. P. FURTH 3,196,649

DEVICES FOR METAL-FORMING BY MAGNETIC TENSION Filed Feb. 16, 1962 2 Sheets-Sheet 1 H. P. FURTH 3,196,649

DEVICES FOR METAL-FORMING BY MAGNETIC TENSION July 27, 1965 2 Sheets-Sheet 2 Filed Feb. 16, 1962 INVENTOR. flxeow 15 677 II I] II. l| l.\ x

United States Patent 0 Kinetics, Inc., Costa Mesa, 'Calif., a corporation of California Filed Feb. 16, 1962, Ser. No. 173,684) 6 (Ilaims. ({Il. 72-56) The present invention relates generally to metal-forming by a pulsed magnetic field, and more particularly to forming devices where a pull is exerted magnetically on the surface of the Work.

An electrically conductive metal work piece may be formed by applying a magnetic pressure against the piece. Typically an electrically conductive transmission line responsive to a high energy current source generates a high strength magnetic field which induces a current in the metal work piece. The interaction between the magnetic field and the current in the work piece creates a magnetic pressure which tends to push the work piece away from the conductor. The metal work piece may then be formed in a desired manner. Since Work is performed by pushing against the metal work piece, the conductor generating the magnetic field must always be located in a position so as to move the metal work piece away from the conductor. Unfortunately in many instances it is not feasible to locate the conductor line in the proper position. Sheets of metal to be formed are often inaccessible from the side where the conductor must be positioned. Tubes and closed metallic vessels are usually inaccessible from the inside making it impractical to expand the tubes by magnetic forming. Accordingly it is an object of this invention to provide a magnetic metal forming device of improved versatility and efiiciency.

The device of the present invention magnetically forms a metal Work piece by exerting a magnetic pressure which pulls the Work piece towards the electrical conductor generating the magnetic field. In this manner magnetic forming may be applied to a variety of Work pieces heretofore inaccessible to the standard magnetic forming techniques.

When a pulsed magnetic field is generated at the surface of the work, the field is transiently prevented from penetrating into the work, by the appearance of induced skin currents. The motor force due to the flow of these induced currents across the applied magnetic field can be used to exert pressure on the work. When the magnetic field pulse rises sufi'iciently slowly, the induced currents are correspondingly weak, and little pressure is exerted during the penetration of the magnetic field into the work. If the applied magnetic field is now rapidly removed, a strong skin current can be induced in the Work, which acts to prevent the removal of the magnetic field from the interior of the work. The direction of this induced current is such that the motor force due to the magnetic field can now be used to exert a pull on the surface of the work, which will be in the direction towards any external coil or circuit linking the magnetic field lines that are passing through the work.

It is therefore another object of this invention to provide a magnetic forming device which forms by magnetic tension.

It is a further object of this invention to provide a magnetic forming device in which a magnetic field is generated for pulling a metal work piece.

It is a still further object of the present invention to provide a number of magnetic forming devices suited to shape metallic work pieces in a useful manner, by magnetic tension.

Other objects and advantages of the present invention ice will become apparent from the following description and appended claims.

In the drawings:

FIGURES 1a and 1b are schematic sectional views of a forming device that is suited to exert a pull on a ringshaped region of the work. The initial configuration FIG. la and the final configuration FIG. lb of the forming process are shown.

FIGURES 2a and 2b are schematics of two alternative electrical circuits that are suitable for operating the primary coils of forming devices that make use of the magnetic tension method. The circuit FIG. 2a is based on the use of an expendable fuse, and the circuit FIG. 2b is based on the use of an auxiliary capacitor bank.

FIGURE3 is a schematic View of a secondary coil that can be used in the forming device of FIGURE 1.

FIGURES 4a, 4b, and 4c are schematic views of forming devices that are suited to exert a pull on a bar-shaped region of the work. A sectional side View of the device is shown in FIG. 4a. Views of the primary coil and of the secondary coil are given in FIGS. 4b and 4c respectivei seen from the rear of the device.

According to a principal aspect of the invention a metal work piece is formed by magnetic tension. A first conductor, responsively connected to an energy source provides a slowly rising magnetic field. An electrically conductive metal work piece is positioned within the slowly rising magnetic field to have a weak current corresponding to the slowly rising magnetic field induced therein. A second conductor is positioned within the magnetic field and is connected to have a current induced therein by a collapsing magnetic field when the first conductor is disconnected from the energy source. The collapsing magnetic field also induces a current in the Work piece which interacts with the current in the second conductor and the collapsing magnetic field to generate a magnetic pressure tending to pull the work piece and the second conductor together. In this manner the Work piece is formed by magnetic pull.

The magnetic tension technique of metal forming makes use of two distinct steps. In the first step, a magnetic field is generated in a primary coil that is held near the surface of the work. This primary magnetic field is made to rise sufficiently slowly so that the associated electromotive force is weak, and only weak currents are induced in the resistive metal of the work. In thi way, little pressure is exerted against the work during the first step, and there is also little expenditure of energy in Joule heating. These conditions will be satisfied, provided that:

If, for example, the work is a tube of radius R and Wall thickness d, the quantity a is to be taken roughly as:

a=(Rd) in the second step of the magnetic tension forming process, the current in the primary coil is interrupted during a time T2 such that A large electrom-otive force is associated with the rapid disappearance of the current and magnetic field of the 3 :33 primary coil. Accordingly, .a large voltage appears across the terminals of the primary coil, and this voltage must be accounted for by a correspondingly large voltage across the circuit element in the primary circuit that causes the interruption of the current. A suitable interrupter element is, for example, a fuse wire in a series with the primary coil, which is designed tomelt at the time when the current interruption is desired. A second example of a suitable interrupter element is a charged auxiliary capacitor bank, which is connected across the terminals of the primary coil at the time when the current interruption is desired. In the process of disappearance of the primary magnetic field, a large electromotive force also appears in the work, giving rise to induced currents that tend to prevent the removal of the primary magnetic field from the interior of the work. If the condition of Equation 4 is satisfied, these induced currents are sufiiciently large so that the trapping of the magnetic field in the work can be made almost totally effective during the time T2. In the process of disappearance of the primary magnetic field, a large electromotive force can also be generated across the terminals of a secondary coil, which is placed next to the primary coil on the side away from the work. The secondary coil is connected to an open circuit during the rise time of the primary magnetic field. An adjustable spark gap may, however, be provided, which is fired by the large electromotive force that is generated at the time of disappearance of the primary magnetic field, and thereafter the secondary coil operates with a closed circuit and can pass current. As an alternative example, the circuit of the secondary coil may be closed by the closing of an external switch, activated by a timing element. Once the circuit of the secondary coil is closed, the magnetic fiux initially linking it tends to be trapped. The secondary circuit is so designed that its resistance i sufiiciently low so that the initial magnetic flux remains almost completely trapped during the time T2. When the current has disappeared from the primary coil, two other currents have thus been set up by induction: a current in the work, and a current in the secondary coil. These two currents are linked by that remaining portion of the initial magnetic field which is trapped both in the work and in the secondary coil. The motor force associated with the flow of the induced currents across the linking magnetic field tends to pull the work and the secondary coil together. The resultant pressures and given roughly by B2 P g where B is the strength of the linking magnetic field in gauss, just below the surface of the work, and the pressure P is in dynes per square centimeter. The magnitude of P must be sutficiently great so that the forming process can be accomplished within a time that satisfies the condition of Equation 4-.

The magnetic tension technique of this invention provides a unique and useful means for applying a strong pull on the surface of a work piece. Some examples of practical applications are the creation of protuberances on sheets of metal that are inaccessible from the backside or on tubes or closed metallic vessels where access from the inside may be inconvenient or impossible. Such protuberances may be shaped against a non conducting mold placed between the work and primary coil.

Referring now to the drawing in the magnetic tension forming device of FIGURE. 1, a primary conductor coil 11 with an arbitrary number of turns 12 generates a pri mary magnetic field 113. For this purpose a slow rising current is set up in the primary circuit 14 by the capacitor bank 15, which is fired by closing the switch 16. The current is interrupted, after peak magnetic field is reached, by the melting of the fuse 17. The configuration just prior to'the melting of the fuse 117 is shown in FIG. la and the configuration that ensues after the melting of the fuse 17a is shown in FIG. 1b. The current in the circuit lid is interrupted by the melting of the fuse, and the magnetic field 13 tends to collapse. The electrornotive force associated with the collapse of the magnetic field 13 induces a ring shaped eddy current in the work 13 in the region under coil 11. The electrom-otive force also causes the firing of the spark gap 19 across the terminals of the secondary coil 29. An induced current then flows in the secondary coil. The induced currents in the work 18 and in the secondary coil 2% are linked by the residual magnetic field 13a after the current in the primary circuit 14 has been interrupted. The motor forces associated with the flow of the induced currents across the linking magnetic field 13:; are such as to pull the work 18 and the secondary coil 26) together, resulting in the formingof the work 180. A non conducting mold 8 may be introduced between the work 1.3 and the primary coil 11 to control the forming process.

FTGURES 2a and 2b show two examples of electrical circuits that are suitable for use in conjunction with the primary coils of magnetic tensionforming devices, such as that of PTGURE 1. In the circuit of FIG. 2a as in FIG. 1, a fuse 17 is employed as the interrupting element. Since the duration of the melting and vaporization processes of the fuse wire is somewhat uncontrollable, a shunt capacitor 21 may be used to prevent the occurrence of extreme voltages during the interruption process. A damping resistorZZa may also be used to control oscillations resulting from the interruption process. In the circuit of 252, an auxiliary capacitor 23 is used as the current interrupting element. This capacitor is charged to a polarity opposite to that of the capacitor 15b and is charged to a much higher voltage, its capacity being much smaller To interrupt the flow of current to the primary coil, the switch 2 is closed. The series inductance 25, Which is comparable in magnitude to the inductance of the primary coil, prevents the short cir-cuiting of the capacitor 23 by the capacitor 15b. The net effect is that the primary current is diverted so as to flow through the capacitor 23 instead of through the primary coil. To prevent the curent in the primary coil from oscillating, a damping resistor (not shown) may be used, or an auxiliary switch may be closed across the terminals of the primary coil at the instant when the current in the primary coil has vanished.

FIGURE 3 shows an example of a secondary coil that may comprise the coil 2% and associated circuit elements to be used in conjunction with the forming device of FIG- URE 1. As in PTGURE l, the secondary coil 29 makes use of a spark gap directly mounted on it. This spark gap has an adjustable screw 2" which permits it to be set at a convenient gap height, such that the spark gap will not fire during the initial rise of the primary magnetic field, but will fire readily when the primary current is rapidly interrupted.

In order to exert a magnetic pull on a bar shaped region of the work, the device of PTGURE$ 4a, 4b and 40 may be used. The primary coil 35, shown in the side view of PEG. 4a and the full view of FIG. 412 gives rise to the magnetic field as, which is strongest over a bar shaped region of the work 33. An external primary circuit, as'in the ex amplcs of FTGURE 2, is attached to the terminals 37, for the purpose of generating and interrupting the primary magnetic field as. A full view of the secondary coil 39 is given in FIG. 40. When the primary current is interrupted, the secondary coil circuit may be closed, for example, by the firing of the spark gap The initial current 42 in the primary coil and the current 42 induced in the secondary coil on interruption of the primary current are indicated. A similar current pattern is induced in the work 38. The magnetic field linking the current in the work and the current in the secondary coilgives rise to motor forces pulling them together and forming the work. A non conducting mol may be placed between the work and the primary coil 35.

Various of the novel features of the present invention are set forth in the following claims.

I claim:

1. In a device for forming metal by magnetic pressure,

energy source means for generating a slowly rising current,

a first conductor responsively connected to said energy source means to provide a slowly rising magnetic field corresponding to said slowly rising current,

an electrically conductive metal work piece positioned within said magnetic field to have a current induced therein corresponding to said slowly rising magnetic field,

a second conductor positioned within said magnetic field,

means for disconnecting said first conductor from said energy source, said magnetic field collapsing when said first conductor is disconnected from said energy source,

said collapsing magnetic field inducing currents in said metal work piece and said second conductor which interact with said magnetic field to create magnetic pressure on said work piece.

2. The device recited in claim 1 wherein said first and second conductors are on the same side of said work 3. A magnetic metal forming device comprising,

energy source means for generating a slowly rising current,

a first conductor,

first switch means for completing a current path from said energy source to said first conductor when closed, whereby a slowly rising magnetic field corresponding to said slowly rising current is generated when said switch means is closed,

an electrically conductive metal work piece positioned within said magnetic field,

said magnetic field generating a weak electromotive force and corresponding weak current in said work piece corresponding to said slowly rising magnetic field,

said slowly rising magnetic field and said weak current interacting to generate a magnetic pressure on said work piece to push said work piece from said first conductor, the magnitude of said magnetic pressure corresponding to said weak current and said slowly rising magnetic field, whereby said magnetic pressure is insufficient to form said work piece,

a current responsive circuit breaker in the current path from said energy source to said first conductor for opening said current path when a predetermined value of said slowly rising current is reached,

a second conductor positioned within said magnetic field on the same side of said metal work piece as said first conductor,

said second conductor having end terminals separated by a spark gap set to close at a predetermined electromotive force,

said magnetic field collapsing when said circuit breaker opens whereby electromotive forces are induced by by said collapsing magnetic field in said metal work piece and said second conductor,

the electromotive forces induced in said work piece generating a corresponding current in said work piece, and the electromotive force induced in said second conductor being of a value to close said spark gap and induce a current in said second conductor,

the currents induced in said metal work piece and said second conductor by said collapsing magnetic field being in a direction to trap said collapsing magnetic field generating a magnetic pressure on said work piece to pull said work piece towards said first and second conductors.

4. The device recited in claim 3 wherein said metal work piece is a tube, and wherein is provided a mold adjacent said tube and between said tube and said conductors.

5. The device recited in claim 3 wherein said second conductor comprises a donut shaped coil of a single turn.

6. In a process for forming an electrically conductive metal work piece the steps comprising,

placing said metal work piece adjacent a first conductor,

placing a mold between said work piece and said first conductor, applying a slowly rising current pulse to said first conductor to generate a corresponding slowly rising magnetic field and a corresponding weak current in said metal work piece, whereby a magnetic pressure is created for pushing said work piece away from said first conductor, said magnetic pressure being too weak to deform said work piece, placing a second conductor adjacent said metal work piece on the same side as said placed first conductor,

removing said current pulse from said first conductor, whereby said magnetic field collapses generating a current in said second conductor, said collapsing magnetic field and said current in said second conductor generating a magnetic pressure on said work piece to pull said work piece towards said first conductor thereby forming said work piece against said mold.

References Cited by the Examiner UNITED STATES PATENTS 2,976,907 3/61 Harvey 113-44 CHARLES W. LANHAM, Primary Examiner. WILLIAM J. STEPHENSON, Examiner. 

1. IN A DEVICE FOR FORMING METAL BY MAGNETIC PRESSURE, ENERGY SOURCE MEANS FOR GENERATING A SLOWLY RISING CURRENT, A FIRST CONDUCTOR RESPONSIVELY CONNECTED TO SAID ENERGY SOURCE MEANS TO PROVIDE A SLOWLY RISING MAGNETIC FIELD CORRESPONDING TO SAID SLOWLY RISING CURRENT, AN ELECTRICALLY CONDUCTIVE METAL WORK PIECE POSITIONED WITHIN SAID MAGNETIC FIELD TO HAVE A CURRENT INDUCED THEREIN CORRESPONDING TO SAID SLOWLY RISING MAGNETIC FIELD, A SECOND CONDUCTOR POSITIONED WITHIN SAID MAGNETIC FIELD, MEANS FOR DISCONNECTING SAID FIRST CONDUCTOR FROM SAID ENERGY SOURCE, SAID MAGNETIC FIELD COLLAPSING WHEN SAID FIRST CONDUCTOR IS DISCONNECTED FROM SAID ENERGY SOURCE, SAID COLLAPSING MAGNETIC FIELD INDUCING CURRENTS IN SAID METAL WORK PIECE AND SAID SECOND CONDUCTOR WHICH INTERACT WITH SAID MATNETIC FIELD TO CREATE MAGNETIC PRESSURE ON SAID WORK PIECE. 